If that code is correct, its saying your drivers side cylinders are running lean as you state. Having some kind of vacuum leak on that side or even an exhaust leak sensed by that upstream sensor could cause it to fault. Even the sensor itself, could have started loosing its ability to cycle its proper voltage signal back to the ECM, creating that fault. Is this a first time event your having with this fault code ? Does the car exhibit any loss of response or power ? If you reset the fault, how soon does it reappear ?